riscv: mm: make pmd_bad() check leaf condition

In the definition in Documentation/vm/arch_pgtable_helpers.rst,
pmd_bad() means test a non-table mapped PMD, so it should also
return true when it is a leaf page.

Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>